require 'date'
leap_year_day = Date.new(1996, 2, 29)
(leap_year_day << 1).to_s # => "1996-01-29"
(leap_year_day >> 1).to_s # => "1996-03-29"
(leap_year_day >> 12).to_s # => "1997-02-28"
(leap_year_day << 12 * 4).to_s # => "1992-02-29"